Coating compositions are often divided into thermoplastic and thermoset compositions. Thermoplastic coating compositions utilize high molecular weight polymers dispersed in an organic or aqueous solvent. After the coating composition is applied to a substrate, the solvent is removed and the polymers is set up to form a film. Thermoset or curable coating compositions utilize two components that are reactive with one another under certain cure conditions. The reactive groups on these components are referred to as `functional groups`. After the composition containing these components is applied, the coated substrate is subjected to cure conditions, causing the functional groups to react and form a cured film of a crosslinked matrix.
Thermoset or curable coating compositions that cure via the formation of urethane bridges in the crosslinked coating are widely used in the art. Urethane bonds are often desirable for their durability, resistance to attack by various agents in the environment, impact resistance, and other physical properties such as stress release. Urethane bridges can be formed by various combinations of functional groups, such as OH functional groups and isocyanate functional groups, carbamate groups and aminoplast resins, or cyclic carbonate groups and amino groups.
In many coating applications, it is often necessary to apply an additional coating on top of an already-cured coating. This may be done to achieve certain visual or physical properties, or it may be necessary to repair areas where the cured coating has been damaged or where coating defects exist. In such cases, it is important that the coating applied on top of the cured coating have adequate adhesion to the cured coating. Sanding the surface generally improves adhesion, but it may not be desirable to sand a surface because of resulting change in the appearance of the sanded surface. Even when the cured coating is sanded prior to application of the additional coating, the adhesion factor is still a concern with regard to overspray areas onto unsanded coating surfaces.
Intercoat adhesion can be of particular concern with regard to coatings that form urethane linkages upon cure, due to the physical and chemical properties of the coating. Intercoat adhesion may be poor due the highly crosslinked nature of the cured film. It has unexpectedly been found that the addition of a polysiloxane compound to a coating composition based on a carbamate-melamine cure system improves intercoat adhesion when compared to a coating of an identical composition without the polysiloxane compound. Additionally, the present invention is directed toward providing curable coating compositions having good intercoat adhesion properties when a cured coated surface is subjected to repair conditions. Repair conditions generally involve application of at least one additional coating followed by application of heat. This result is unexpected, since polysiloxanes which are usually used to improve flow and leveling and to improve mar resistance, generally adhere poorly to subsequent coatings, as described in U.S. Pat. No. 4,812,518.
It is also desirable to promote and/or improve adhesion of basecoat and/or the clearcoat coating compositions to sealant materials, such as where windshield or other sealants are applied to a cured coating. Accordingly, the present invention is also directed toward a cured coating composition having good adhesion to sealant materials.